1. Field of the Invention
The present invention relates to a process for producing a magnetic head with an appropriately oriented easy axis of magnetization and to a head obtained by this process. The invention is used in magnetic recording. The head obtained by the process according to the invention can either be of the writing and reading, or the reading or writing type.
2. Discussion of the Background
FIGS. 1 and 2 show an embodiment of such reading and writing magnetic heads for narrow recording tracks. The magnetic head 10 is located on the lateral face 12 of a support 14 having a generally catamaran shape. The recording medium carries the reference 16 and its width is approximately 2 .mu.m.
The head essentially comprises a magnetic circuit having a first part constituted by two pole pieces PP1, PP2 (e.g. of FeNi) of limited thickness (e.g. 2 .mu.m) separated by a very narrow air gap G (e.g. 0.3 .mu.m). These pole pieces form a U-shaped pattern with two "vertical" branches BV1, BV2 (i.e. having a direction generally perpendicular to the plane of the recording support), said branches being relatively wide (a few tenths of a mm) and a "horizontal" branch BH (i.e. parallel to the recording medium) and which is much narrower (approximately 1/10 mm).
The magnetic head comprises a second part to enable the magnetic flux to form a closed loop. In FIG. 2, said second part is in the form of a ferrite bar 20 bonded at its two ends to the pole pieces. This bar is surrounded by a conductor coil 22, e.g. made from copper and with a diameter of 15 .mu.m. Such a head is able to read and write on the medium 16.
There are numerous other types of heads, e.g. with a coil formed from thin films deposited by microelectronics methods as described in FR-A-2 604 021, or read only heads with magnetosensitive elements inserted in the magnetic circuit and as described e.g. in FR-A-2 612 676 or EP-A-0 269 129.
As the invention essentially relates to the production of the first part of the magnetic circuit, namely the U-shaped part with two pole pieces and an air gap, there is no need to describe all these variants, that shown by FIGS. 1 and 2 being adequate for understanding the problem solved by the invention.
FIG. 3 gives the essence of the functions fulfilled by such a head. The head is placed in front of a track on medium 16 having a magnetization able to assume either of two directions. The magnetic induction in the pole pieces PP1 and PP2 is represented by arrows. This induction is closed by a rear part 25 (not shown). The induction is substantially horizontal in the narrow horizontal branch BH (i.e. parallel to an axis x) and substantially vertical in the two wide vertical branches BV1, BV2 (i.e. parallel to an axis y).
In order to reduce the noise in the electric reading signal and obtain a good efficiency of the head, the easy axis of magnetization of the magnetic layer forming the pole pieces (magnetization obtained in the absence of any external field) must be perpendicular to the induction obtained in the presence of a recording support, in order to best take advantage of the coherent rotation phenomenon of the magnetization in the magnetic layer.
In the inoperative state, i.e. remote from any recording medium, the magnetization would ideally have the configuration shown in FIG. 4, with a generally vertical direction (parallel to y) in the narrow horizontal branch BH and around the air gap G and a generally horizontal direction (parallel to x) in the two wide vertical branches BV1, BV2.
When using without particular precautions the known processes for producing such heads and e.g. processes using electrolytic deposits, as described in U.S. Pat. No. 4,402,801 or EP-A-0 262 028, it is not possible to obtain this ideal situation. It is even possible to obtain the opposite situation as is shown in FIG. 5, which illustrates a few stages in the production of such a head.
The following operations are carried out in known manner:
on a non-conductive substrate 30 is deposited a metal layer 32 and then a resin 34 (FIG. 5a); (if the substrate is conductive or semiconductive there is no need for the metal layer);
the resin is exposed and developed to obtain a step 36 (FIG. 5b);
this is followed by the deposition of an nonmagnetic material layer 38, e.g. of silica (FIG. 5c);
by reactive ionic etching removal takes place of the horizontal portions of the layers 36 and 38 so as to only leave an amagnetic wall 40 (d);
this is all covered with resin 42 (e);
in the resin is etched a trench 44 shaped like a U-shaped pattern and which is intended to constitute the future pole pieces (f in plan view and g in section through the wall 40);
an electrolytic deposit 46 is formed in the trench, e.g. of FeNi, in the presence of an external field Hext (h);
the remainder of the resin is made to disappear and the U-shaped circuit is obtained with its two pole pieces PP1 and PP2, the amagnetic spacer 40 being in the air gap G (i,j).
In order to attempt to obtain an easy axis of magnetization parallel to the air gap, Hext is directed parallel to the wall 40. Although the easy axis of magnetization is parallel to Hext in the vertical branches of the U-shaped pole pieces PP1 and PP2, due to a very marked edge effect creating an anisotropy in the narrow horizontal branch the easy axis of magnetization switches and is parallel to the horizontal edges. The result obtained (FIG. 5j) is consequently the opposite to the desired result (FIG. 4).
On switching or tilting the external field Hext by 90.degree., the desired orientation would indeed be obtained in the wide branches, but the orientation would a fortiori be poor in the vicinity of the air gap. However, it is in this narrow zone that the magnetic phenomena are the most intense and require an appropriate orientation of the easy axis of magnetization.